Portable phones that cater to demands for a wider screen, a higher resolution, and lower power consumption include those on which an LCD panel having a VGA size (640×480 pixels) is mounted. The controlling unit of such a portable phone selects either a VGA mode in which image are displayed on the whole LCD panel based on display data for VGA size (640×480 pixels) or a QVGA mode in which images are displayed on the whole LCD panel based on display data for QVGA size (320×240 pixels), and displays images on the LCD panel at the resolution of the selected mode.
In the QVGA mode, the area (the number of pixels) over which an image is displayed based on a display data is four times as large (twice as large in both the vertical direction and the horizontal direction) as that in the VGA mode. Therefore, the QVGA mode is also called four times enlarging mode.
The VGA mode requires four times as many display data as required in the QVGA mode. Hence, the VGA mode imposes a heavier processing load on the driver that drives the LCD panel and on the Central Processing Unit (CPU) that controls the LCD panel and the driver, and causes them to consume more power. Therefore, it is preferable that the mode selection should switch to the VGA mode when the higher resolution is required, and to the QVGA mode when the higher resolution is not required, that is, when it is hard for the user to feel the effect of the VGA mode. The following will explain a first conventional portable phone and a second conventional portable phone, both of them display images on the LCD panel at the resolution of a selected mode.
The first conventional portable phone includes: an LCD panel that displays images corresponding to supplied serial data at the resolution of a designated mode; a parallel/serial converting circuit that converts supplied parallel data to serial data and supplies it to the LCD panel; an LCD controller that converts supplied data to parallel data corresponding to the resolution of a designated mode and supplies the data to the parallel/serial converting circuit; and a control unit that supplies data to the LCD controller, and supplies a signal that designates a mode to the LCD panel, the parallel/serial converting circuit, and the LCD controller.
The parallel/serial converting circuit is prepared for reducing the number of signal lines between the LCD controller and the LCD panel. The parallel/serial converting circuit takes a time period of 1 VBlank (one frame cycle) from when it is instructed to switch the modes until when it finishes the switching.
The second conventional portable phone has basically the same configuration as the first conventional portable phone, except that it has no parallel/serial converting circuit. Specifically, the second conventional portable phone includes: an LCD panel that displays images corresponding to supplied data at the resolution of a designated mode; an LCD controller that converts supplied data to data corresponding to the resolution of a designated mode and supplies it to the LCD panel; and a control unit that supplies data to the LCD controller, and supplies a signal that designates a mode to the LCD panel and the LCD controller.
However, note that the LCD controller of the second conventional portable phone, unlike the LCD controller of the first conventional portable phone, has a circuit that takes a time period of 1 VBlank from when it is instructed to switch the modes until when it finishes the switching.
The first conventional portable phone and the second conventional portable phone will cause a flicker on the screen of the LCD panel, unless they simultaneously switch, when switching the modes, the data and the mode designating signal both to be supplied to the LCD panel, because a failure to simultaneously switch them spoils the correspondence between the data and the mode. However, as described above, since some time is taken from when the parallel/serial converting circuit or the LCD controller is instructed to switch the modes until when it finishes the switching, there is a gap between the timing at which the data to be supplied to the LCD panel is switched and the timing at which the mode designating signal to be supplied to the LCD panel is switched, and the screen of the LCD panel thus flickers.
To solve this problem, the first conventional portable phone performs, for example, a liquid crystal display process shown in the flowchart of FIG. 5. Note that a liquid crystal display process performed by the second conventional portable phone is a changed version of the flowchart shown in FIG. 5, in which changing the setting of the parallel/serial converting circuit (step S103) is skipped, and waiting for one cycle to reflect the setting (step S104) comes after switching the LCD controller to the QVGA mode (step S106). Here, to avoid repetitive description, the liquid crystal display process performed by the first conventional portable phone will only be explained.
The control unit controls the LCD panel to display images in the VGA mode (step S101). Next, the control unit turns OFF the display on the LCD panel so as not to flicker the screen during mode switching (step S102).
Next, the control unit issues an instruction to change the setting of the parallel/serial converting circuit to the setting for the QVGA mode (step S103). Then, to reflect the setting, the control unit waits for one cycle. (1 VBlank period) until the setting change is completed in the parallel/serial converting circuit (step S104).
Next, the control unit switches the LCD panel to the QVGA mode (step S105). Further, the control unit switches the LCD controller to the QVGA mode (step S106).
Next, the control unit turns ON the display on the LCD panel (step S107). Then, the control unit controls the LCD panel to resume display in the QVGA mode (step S108). Since the control unit turns the screen of the LCD panel OFF (black) while switching modes at steps S103 to S106, the screen can be prevented from flickering.
Patent Literature 1 discloses a liquid crystal display device that prevents flickers on the screen by a method different from those taken by the first conventional portable phone and the second conventional portable phone. The liquid crystal display device disclosed in Patent Literature 1 is provided with a switching element in each pixel defined at the intersections where a plurality of scanning electrodes and a plurality of signal electrodes meet, and scans through the scanning electrodes to select them sequentially by a first drive circuit while supplying an image signal via the switching elements to the pixels corresponding to the selected scanning electrodes from a second drive circuit via the signal electrodes. The device stops the scanning operation for a certain frame period, and during this stopping period, displays the image of the frame before the stop. Hence, it is possible to realize a liquid crystal display device that uses no frame memory to make no disturbed image show up on the screen.
Further, Patent Literature 2 discloses a display device that prevents flickers on the screen by a method different from those taken by the first conventional portable phone, the second conventional portable phone, and the liquid crystal display device disclosed in Patent Literature 1. The display device disclosed in Patent Literature 2 switches modes between a normal power consumption mode and a low power consumption mode. In the normal power consumption mode, the device displays display data supplied by a display controller on a liquid crystal display element, while in the low power consumption mode, the device retains display data supplied by the display controller in a memory, stops the display controller, and then displays the display data retained in the memory on the liquid crystal display element. The device has a flicker prevention circuit that stops the display on the liquid crystal display element when the normal power consumption mode and the low power consumption mode are switched.
Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. 2002-244610
Patent Literature 2: Japanese Patent No. 2941409